The present invention relates to generally a device for fluoride iontophoresis on the teeth of not only human beings but also other various forms of life for avoiding dental caries and more particularly a device for fluoride iontophoresis on the teeth of a baby, infant or children for avoiding dental caries of deciduous and young permanent teeth.
In general, the more quantity of sugar one takes, the more one is easily susceptible to dental caries. The incidence of dental caries is relatively high among children. Brushing teeth is the oldest and simplest method for avoiding dental caries, but it is difficult to form the habit of brushing teeth in children having the highest incidence of dental decay. Therefore the national statistics still shows a high percentage of incidence of dental caries. One of the various methods so far investigated for reducing the high incidence of dental caries in children and in young adolescents is to improve the tooth's substance and especially the surface layer of the tooth. That is, the body of the tooth consists of dentin which is mainly composed of calcium phosphate and which is covered by enamel in the crown. The enamel consists of prisms and interprismatic substances and is mainly composed of calcium phosphate (hydroxy apatite). The enamel is by far harder and physically and chemically more stable than the dentin. The susceptibility to dental decay is dependent upon one's age, physical constitution and living conditions (including diet). In general, it is considered that dental caries are started by the decalification of the enamel by bacteria produced acids. Dental caries has been defined as a gradual, progressive destruction of the hard portions of the teeth starting from decalification. The hard portions of the teeth may be made resistant to calicification by growing dense crystals of calcium phosphate or by converting the hard portions into crystals of fluoro apatire.
To this end, the introduction or injection of fluoride into the enamel of the teeth is very effective for preventing dental caries. In practice, aqueous solutions and gels of sodium fluoride and stannous fluoride have been used. That is, these solutions or gels are subjected to hydrolysis so that fluorine may be absorbed into the teeth in the form of ions. The uptake of fluoride ions not only help grow the dense crystals of calcium phosphate but also improves the tooth's substance so that the enamel may not be liable to dental caries. Thus dental decay prevention may be attained.
In order to add fluoride into the teeth, a small amount of fluoride may be added to communal drinking water; a patient may hold in his mouth an aqueous solution of sodium fluride or stannous fluoride; an aqueous solution of sodium fluoride having a relatively high concentration of 1-3% may be applied to the teeth; and fluoride ions may be electrically absorbed into the teeth. Of these methods, the electrical absorption method is most favorable.
The prior art method for iontophoretic application of fluoride ion into the teeth is such that an aqueous solution of sodium fluoride or stannous fluoride is subjected to electrolysis to produce negative fluoride ions which are absorbed into a tooth which is maintained at a positive potential as an anode. In general, the human body is mainly composed of various water soluble electrolytes so that it is considered that the passage of current through the body is the movement of ions or the ion conduction. The use of direct current for the movement through a substance of ions causes the physical and chemical changes of the substance and the electrochemical polarization so that when the current is used, polarization occurs. The polarization is such that the substances produced on the electrodes by hydrolysis cause a chemical reaction which is just the reverse of the hydrolytic reaction. Furthermore the use of smooth direct current results in variation in concentration of electrolyte so that a counterelectromotive force may be produced. Thus as a result of the polarization or counterelectromotive force, the direct current rises its peak at the instant when it starts to flow, and then decreases exponentially in time. The equivalent circuit of the tooth constructed based upon the observation of the behavior of the direct current in the tooth has a time constant or CR parameter. When a direct current is made to flow through this circuit, it immediately reaches its peak, then abruptly decreases and gradually approaches a flat and stable current. Same is true for the passage of direct current through the human body. That is, the direct current flowing through the body decays exponentially so that it takes an impracticably long time before a desired quantity of fluoride ions is absorbed into the tooth.
According to the experiments conducted by the inventors, the magnitude of the initial current or peak is a few times as high as the stable current. Therefore when a direct current is made to flow through the human body, the polarization which results in such a high ratio of the peak current to the stable current must be taken into consideration. In other words, the effects of the passage of direct current through the human body on the various nerve systems must be taken into consideration. Thus for the electrical uptake of fluoride ions by the teeth a method for flowing a direct current through the human body must be employed which will not only avoid of the injuries to the human body but also result in the efficient uptake of fluoride ions by the teeth.
In order to avoid the polarization, in general an alternating current has been used which reaches maximum in one direction, decreases to zero, then reverses itself and reaches maximum in the opposite direction. However when the aqueous solution of sodium fluoride is subjected to hydrolysis or dialysis, sodium fluoride is dissociated into positive sodium ions Na.sup.+ and negative fluoride ions F.sup.-. The negative fluoride ions F.sup.- are absorbed into a tooth which is maintained at a positive potential as an anode. The use of the alternating current may eliminate the polarization because the alternate change in polarity of electrodes, but the alternating current also causes the alternate change in direction of movement of fluoride ions so that the fluoride ions cannot be accumulated in the tooth. Thus the use of the alternating current in a method for electrical uptake of fluoride ions by the teeth is inpracticable.